The explosion of the Internet and the desire to provide multiple communications and entertainment services to end users have created a need for a broadband network architecture that improves access to end users. One broadband network architecture that improves access to end users is a point-to-multipoint passive optical network (PON). A point-to-multipoint PON is an optical access network architecture that facilitates broadband communications between an optical line terminal (OLT) and multiple remote optical network units (ONUs) over a purely passive optical distribution network. A point-to-multipoint PON utilizes passive fiber optic splitters and couplers to passively distribute optical signals between the OLT and the remote ONUs.
FIGS. 1A and 1B illustrate the management of network traffic in a point-to-multipoint PON. As an example, the PON is shown to include an OLT 102 and three ONUs 104, 106 and 108, although the PON may include additional ONUs. Referring to FIG. 1A, the OLT includes an optical transmitter 110 that sends downstream traffic containing ONU-specific information blocks 1, 2 and 3 to the ONUs. The downstream traffic is optically broadcasted by a passive optical splitter 112 into three separate signals that each carries all of the ONU-specific information blocks. The ONUs 104, 106 and 108 include optical receivers 114, 116 and 118, respectively, that receive all the information blocks transmitted by the OLT. Each ONU then processes the information blocks that are intended for that ONU and discards the information blocks that are intended for the other ONUs. For example, ONU-1 receives information blocks 1, 2, and 3. However, ONU-1 only delivers information block 1 to end user 1. Likewise, ONU-2 only delivers information block 2 to end user 2 and ONU-3 only delivers information block 3 to end user 3.
Referring to FIG. 1B, the ONUs 104, 106 and 108 also include optical transmitters 120, 122 and 124, respectively, to transmit upstream traffic to OLT 102. The upstream traffic is managed utilizing time division multiplexing, in which specific transmission time slots are dedicated to individual ONUs. The ONU-specific time slots are synchronized so that upstream information blocks from the ONUs do not interfere with each other once the information blocks are combined onto the common fiber. For example, ONU-1 transmits information block 1 in a first ONU-specific time slot, ONU-2 transmits information block 2 in a second ONU-specific time slot, and ONU-3 transmits information block 3 in a third ONU-specific time slot. The time division multiplexed upstream traffic is then received by an optical receiver 126 of the OLT.
There are a number of factors that contribute to the efficiency of a point-to-multipoint PON. One such factor is the length of guard bands between combined information blocks of the upstream traffic. These guard bands, or dark spaces, provide safety zones between information blocks to prevent collision of adjacent information blocks when they are combined onto the common fiber. However, the guard bands can occupy a significant amount of bandwidth and consequently, reduce the overall bandwidth of the PON for data transmission. Thus, minimizing the length of the guard bands will increase the bandwidth of the PON. However, in order to reduce the length of the guard bands, the optical transmitters of the ONUs must be able to more quickly start and stop sending optical signals, i.e., faster enable and disable times, to ensure that combined upstream information blocks are properly separated by the guard bands.
Another factor that contributes to the efficiency of the PON is the speed with which the optical transmitters of the ONUs can emit binary optical signals. That is, the speed with which the optical transmitters can modulate between “1” signals and “0” signals. Using optical transmitters with increased modulation speed, the PON can increase the rate with which data is transmitted between the OLT and the ONUs.
Still another factor that contributes to the efficiency of the PON is the amount of light leakage from disabled optical transmitters of the ONUs. Any leakage of light from a single disabled optical transmitter may increase the background noise. This is critical in a point-to-multipoint PON where light leakage from one disabled ONU will combine with light leakage from other disabled ONUs of the PON. The overall effect is that significant background noise may be created by the combined light leakage, which may affect the OLT from differentiating legitimate data from the background noise. This effect is amplified as the number of ONUs supported by the PON is increased.
In view of these factors, there is a need for an optical transmitter with fast enable and disable times, increased modulation speed, and reduced light leakage.